When a vehicle is driving with insufficient tire pressure, fuel efficiency and driving stability may be reduced, and a tire may become damaged while the vehicle is driving at a high speed over a long period of time. The installation of a tire pressure monitoring system (TPMS), which outputs a warning message to a driver when the tire pressure is insufficient, is now common.
Existing tire pressure monitoring systems include a pressure sensor and a wireless module inside in the tire. The wireless module transfers a tire pressure state to a receiver apparatus inside the vehicle in real time. To perform the above operation, the wireless module requires a battery inside the tire. The wireless module is expensive and the tire needs to be disassembled to replace the battery of the wireless module every 60,000 km regardless of the tire wear.
An indirect type pressure measuring schemes can also be used. In the indirect type measuring scheme, a natural frequency of a tire can be found using a speed sensor of a wheel. The method may give unsatisfactory results based on the age condition of the tire. Therefore the direct type measuring scheme, in which an air pressure sensor is included in the wireless module installed inside the tire, has been used in most vehicles.